[#469] cmd/neofs-node: Don't send UpdateState tx at shutdown by default

if `shutdown.offline.enabled` config option set to true, then
send netmap.UpdateState(offline) tx at the shutdown. This config
option is not set by default.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

cmd/neofs-node/main.go

@@ -85,7 +85,9 @@ func shutdown(c *cfg) {
 		closer()
 	}
 
-	goOffline(c)
+	if c.cfgNetmap.goOfflineEnabled {
+		goOffline(c)
+	}
 
 	c.wg.Wait()
 }